Sometimes an automobile moves through a strange region while the driver or a passenger is following the present location on a road map. There exists an apparatus which automates manual operations of this type. This apparatus comprises a map display unit, a detector for detecting the orientation of the automobile, and a means for detecting the distance traveled. After the starting location is entered and the vehicle is started, the present location of the automobile is constantly presented on the map display unit.
In this case, the information presented on the map display unit is only the present location of the automobile. Therefore, the driver or a passenger must discover a road extending to the target location.
In an attempt to solve this drawback, some apparatus have been proposed in Japanese Patent Laid-Open Nos. 195,113/1984 and 195,790/1984. When any one of these apparatus are used, an intended route is manually entered before the departure. This intended route is displayed on a map display unit, together with the present location of the automobile. These apparatus dispense with or simplify the search for the optimum route while the automobile is running. This enables the automobile to go more smoothly.
Some attempts have been made to automate the setting of the intended route, for providing more useful apparatus. An apparatus of this kind is disclosed in Japanese Patent Laid-Open No. 11,499/1985. In this apparatus, every point in a network of roads is regarded as a destination. An apparatus installed on the automobile calculates the shortest distance from every accessible point to each destination. The calculated distance is stored in a memory installed on the automobile. Whenever the automobile crosses a road junction, requisite information is read from the memory and displayed.
Although the detected area is restricted, it is necessary to search the area for the information about every road contained in the area, i.e., interconnection between junctions and distances between them. Then, data about the shortest route to the destination must be stored. For this reason, an exorbitant amount of calculation and a large capacity of memory are needed.